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Abstract

The structure and magnitude of noise correlations in the monkey visual system has been subject to intense debate over the last couple of years. We previously found that neural responses to repeated presentations of the same visual stimulus were close to independent in V1 of awake, fixating monkeys (average rsc: 0.01). Other labs, in contrast, found average levels of correlations up to an order of magnitude higher. Although a number of possible explanations for this discrepancy have been put forward, only few of them have been directly addressed. We tested one of our original hypotheses, that fluctuations of global brain state under anesthesia may induce positive correlations between neurons, which are absent during wakefulness. We performed multi-tetrode recordings in V1 of opiod-anesthetized monkeys under conditions otherwise identical to our previous awake recordings. Activity in anesthetized monkey V1 was dominated by strong coordinated fluctuations involving nearly every active neuron. These state fluctuations evolve on a timescale of 1–2 Hz, substantially slower than what would be expected from shared sensory noise, and resemble up and down states, which have been described for many other types of anesthetics before. During wakefulness, in contrast, such state fluctuations were absent. We further found that after accounting for the brain state under anesthesia the level of noise correlations was reduced to that during wakefulness. Our results highlight an important caveat of neural population recordings under anesthesia: if not properly accounted for, state fluctuations, which are not present in awake animals, are the primary source of correlated variability.